ON PLANT ('O.MPOSITKtN AND MANM IUAI. ]{!':( H' HI I-IMKNTS. 113 
piled from my own analyses, which gives the actual composition of one 
Uilogramme of each plant taken in its normal vegetative condition, 
enables us, after having confirmed the practical results under cultivation, 
to fonnulate the second tabl(>, which is a rrsuvu'. of the practical aliment- 
ary requisites of a certain number of vegetables. 
It will be noticed, at the first glance, that water is the most important 
factor in vegetable production. Without water no vegetation is possible. 
Physiologists, and especially ^1. Duhi'rain, have calculated that to pro- 
duce one kilogramme of dry ^ egetable matter it would be necessary that 
there should be in that vegetable a circulation of water of not less 
than 250 litres. To make the point clear, take the Milan Cabbage as 
an example. We find that a crop of 50,000 kilogrammes of Cabbage 
per hectare has necessitated the evaporation of a minimum of 1,175,000 
htres of water ; which represents a rainfall in one year of 80 centimetres. 
This brings into evidence the fact that in dry climates the crops are always 
proportionate to the /quantity of rain, and shows also the enormous im- 
portance of the irrigation question in all horticultural operations. 
We find that the various crops withdraw from the soil very varying quan- 
tities of the different useful elements. For example, whilst 2(),000 kilos, of 
Potatos only take up 24 kilos, of nitrogen, we find that a heavy crop of 
Turnips absorbs nearly 7H kilos, of that element. If we consider the case 
of phosphoric acid and potash, we find differences which are often even 
more considerable still. This difference, from the point of view of the 
elementary needs of different plants, has given rise to the practice of 
rotation, which consists in growing in succession to a plant requiring 
much nitrogen a plant which requires less, and the same for other 
elements. This rule of rotation, established by judicious practice based 
on observation of cultural facts, is found to be fully justified by scientific 
study of the question. We know also now that the various mineral 
elements in the soil are in a condition of assimilation which varies with 
time. The analysis of a soil may at one time demonstrate it to be rich in 
all materials useful to plants, and yet after some crops have been taken 
off, the soil appears to be exhausted. This fact in former times led to the 
obligation of fallow periods : the land was left untilled, a part of the 
useful elements became, after some time, again capable of being 
assimilated, and a fresh crop could then be raised. We can now dispense 
with these costly practices, which rendered large areas useless, causing a 
dead loss ; by means of soluble complementary manures it is easy for us 
either to facilitate the solution of certain mineral constituents of the soil, 
or even to furnish artificially these same elements to the roots of the 
plants. 
The Soil the Souece of Pla^t Aliment. 
Nitrogen. — Plants find their nitrogenous food in the soil under the 
form either of ammoniacal salts, nitrates, or nitrogen combined with 
organic matter. We know that in the first two of these forms the nitrogen 
is eminently absorbable by plants. As for the third, we think with Sachs 
that plants can absorb a certain proportion of nitrogen combined with 
carbon in the components of humus. This seems to us justified by horti- 
cultural practice, wherein soils as rich as possible in humus are always 
selected for all cultures. 
